A marine thruster is a transversal propulsion device built into or mounted on a ship, boat or underwater vehicle. Thruster-based propulsion is relied upon for station keeping, attitude control, and other special propulsion needs. Submersible vehicles such as those used for subsea research and other marine operations may employ one or more thrusters as their principal form of propulsion and maneuverability. Thrusters on such are often powered by electric motors, but space and weight capacity may place constraints on allowable battery size or weight. Thus, in order to maximize the time the vehicles may remain submerged and operating, power conservation is an important consideration. Accordingly, it is desirable to produce high efficiency thrusters which consume less power than those previously known in the art.
The battery powered electric motors used in thrusters for submerged marine applications generally comprise a Rotor/Stator combination. The rotor is the revolving, typically inner portion of an electric motor. It turns due to torque generated as a result of the induction of a fluctuating magnetic field as current flows through surrounding stator windings. In actual operation, the rotor speed always lags the magnetic field's speed, causing the rotor bars and/or surfaces to cut magnetic lines of force and produce useful torque. The difference between the synchronous speed of the magnetic field and the shaft rotating speed is slip, and can be expressed as a differential RPM (revolutions per minute) or frequency.
There are a number of factors which cause losses in efficiency. Windage and friction losses may dominate power loss in certain constructions. Power loss in some induction motors is largely but not entirely proportional to the square of the slip and may be reduced by decreasing the degree of slip for a given load. Implementation of such efficiencies has traditionally been accomplished by increasing the mass of the rotor conductors (conductor bars and end-plates) and/or increasing their conductivity and to a lesser extent by increasing the total magnetic field across the gap between rotor and stator.
The stationary part of an electric motor is referred to as the stator. While various designs and geometries exist, exemplary stators have a rigid (usually steel) frame enclosing a hollow cylindrical core (often made up of laminations of silicon steel to reduce hysteresis and eddy current losses). The stator may be either a permanent magnet or an electromagnet.
Energy is provided to the stator though wrapped wire known as the field coil or field windings. The wire of the windings is generally laid in coils wrapped around core of the stator to form magnetic poles in response to current flow through the wire. Commutation, or polarity reversal of the coils, may be effected through electromechanical means or through commutated induction and permanent magnet motors.
The space between the rotor and stator in the assembled motor is known as the gap. In non-liquid-filled motors, the gap is often referred to as the “air gap”. It is preferred that the gap between the rotor and stator be as small as possible to minimize slip. However, to the extent that the rotor may wobble (e.g. not define a perfect circle) as it turns, the gap must be wide enough to ensure physical contact does not occur between the rotor surface and the stator. A too-large gap distance may have a strong negative effect on motor efficiency.
The efficiency of a motor is determined by intrinsic losses that can potentially be minimized through appropriate motor design. Intrinsic efficiency losses are of two types: fixed losses, which are independent of motor load, and variable losses, which are dependent on load. Fixed losses include magnetic core, friction and windage losses. Magnetic core losses (sometimes called iron losses) consist of eddy current and hysteresis losses in the stator. They vary with the core material and geometry and with input voltage. Friction and windage losses are caused by friction in the bearings of the motor and aerodynamic losses associated with part rotation.
Variable losses include resistance losses in the stator and in the rotor and miscellaneous stray losses. Resistance to current flow in the stator and rotor results in heat generation that typically is proportional to the resistance of the material and the square of the current (I2 R). Stray current losses arise from a variety of sources and are difficult to either measure directly or to calculate, but are generally proportional to the square of the rotor current. Part-load performance characteristics of a motor also depend on its design. Both motor efficiency η and power factor PF fall to very low levels at low loads.
Marine thruster efficiency is commonly expressed in terms of the amount of useful work capable of being performed for a given amount of energy input; work is defined as force times distance, where force may be measured in linear (e.g. forward motion or velocity) terms or as torque. For the purpose of this disclosure, “end-to-end efficiency” is defined as work done for the amount of electric power input.
Submerged marine vehicles for commercial, research and/or military use, such as AUVs (autonomous underwater vehicles) or ROVs (remotely operated vehicles), are often operated at speeds ranging from approximately 10 centimeters per second up to about 4.5 meters per second. Typical end-to-end efficiencies of marine thrusters for AUV/ROVs seldom exceed peak values of 40 percent at any point in the entire rpm/velocity range. Values are generally on the order of 20 percent or less with peak values only occasionally exceeding 35 or 40 percent for discrete portions of the velocity range. Curves 12 and 14, FIG. 6, are depictions of typical thruster end-to-end efficiencies across the rpm/velocity range. As described in more detail below, curve 16 is representative of the surprising and unexpected end-to-end efficiency according to the present invention.
Historical approaches to extending mission endurance for battery powered vehicles often employed either small diameter brushless DC thrusters with gear-boxes (Whitcomb et al., Navigation and Control of the Nereus Hybrid Underwater Vehicle for Global Ocean Science to 10,903 m Depth: Preliminary Results. In Proceedings of the 2010 IEEE International Conference on Robotics and Automation; Gomez-Ibanez et. al., Energy Management for the Nereus Hybrid Underwater Vehicle, in Oceans 2010, IEEE), or used large diameter direct drive disk motors. Each approach has specific drawbacks. Small diameter brushless motors and gear-boxes may suffer from windage losses if run in oil. When designed for low pressure operation (1 atmosphere), small-diameter brushless motors and gear-boxes often require heavy housings and magnetic clutches. Such thrusters are often acoustically noisy and gear-boxes are a source of unreliability. Disk motors, on the other hand, are only useful when their larger size (disk rotors are generally greater than 6 inches in diameter, especially for higher torque applications) is acceptable, such as in single thruster applications for large diameter torpedo-shaped vehicles.
Accordingly, it is desirable to produce low-profile thrusters with improved efficiency and reduced power consumption compared to those known in the art.